5.8 Preparation of Alkenes: Elimination Reactions

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5.8 Preparation of AlkenesElimination Reactions
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b-Elimination Reactions Overview

• dehydrogenation of alkanes X Y H
• dehydration of alcohols X H Y OH
• dehydrohalogenation of alkyl halides X H Y
  Br, etc.

Y
X
a
b
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Dehydrogenation

• limited to industrial syntheses of ethylene,
  propene, 1,3-butadiene, and styrene
• important economically, but rarely used in
  laboratory-scale syntheses

750C
CH3CH3
750C
CH3CH2CH3
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5.9Dehydration of Alcohols
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Dehydration of Alcohols
H2SO4

H2O
CH3CH2OH
160C
H2SO4

H2O
140C
(79-87)

H2O
(82)
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Relative Reactivity
tertiarymost reactive
primaryleast reactive
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5.10Regioselectivity in Alcohol DehydrationThe
Zaitsev Rule
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Regioselectivity

90
10

• A reaction that can proceed in more than one
  direction, but in which one direction
  predominates, is said to be regioselective.

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Regioselectivity

84
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• A reaction that can proceed in more than one
  direction, but in which one direction
  predominates, is said to be regioselective.

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The Zaitsev Rule

• When elimination can occur in more than one
  direction, the principal alkene is the one
  formed by loss of H from the b carbon having
  thefewest hydrogens.

three protons on this b carbon
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The Zaitsev Rule

• When elimination can occur in more than one
  direction, the principal alkene is the one
  formed by loss of H from the b carbon having
  thefewest hydrogens.

two protons on this b carbon
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The Zaitsev Rule

• When elimination can occur in more than one
  direction, the principal alkene is the one
  formed by loss of H from the b carbon having
  thefewest hydrogens.

only one proton on this b carbon
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The Zaitsev Rule

• When elimination can occur in more than one
  direction, the principal alkene is the one
  formed by loss of H from the b carbon having
  thefewest hydrogens.

only one proton on this b carbon
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5.11Stereoselectivity in Alcohol Dehydration
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Stereoselectivity

• A stereoselective reaction is one in which a
  single starting material can yield two or more
  stereoisomeric products, but gives one of them
  in greater amounts than any other.

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5.12The Mechanism of Acid-CatalyzedDehydration
of Alcohols
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A connecting point...

• The dehydration of alcohols and the reaction of
  alcohols with hydrogen halides share
  thefollowing common features
• 1) Both reactions are promoted by acids
• 2) The relative reactivity decreases in
  the order tertiary gt secondary gt primary
• These similarities suggest that carbocations
  areintermediates in the acid-catalyzed
  dehydration ofalcohols, just as they are in the
  reaction of alcoholswith hydrogen halides.

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Dehydration of tert-Butyl Alcohol

H2O

• first two steps of mechanism are identical
  tothose for the reaction of tert-butyl alcohol
  withhydrogen halides

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Mechanism
Step 1 Proton transfer to tert-butyl alcohol
..



O

• (CH3)3C

H
fast, bimolecular



O
H
tert-Butyloxonium ion
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Mechanism
Step 2 Dissociation of tert-butyloxonium
ion to carbocation
slow, unimolecular


(CH3)3C
tert-Butyl cation
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Mechanism
Step 3 Deprotonation of tert-butyl cation.

fast, bimolecular


H
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Carbocations

• are intermediates in the acid-catalyzed
  dehydration of tertiary and secondary alcohols
• carbocations can
• react with nucleophileslose a b-proton to form
  an alkene

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Dehydration of Primary Alcohols
H2SO4

H2O
CH3CH2OH
160C

• avoids carbocation because primary carbocations
  are too unstable
• oxonium ion loses water and a proton in
  abimolecular step

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Mechanism
Step 1 Proton transfer from acid to ethanol
..


CH3CH2
O
H
fast, bimolecular
H



O
CH3CH2
H
Ethyloxonium ion
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Mechanism
Step 2 Oxonium ion loses both a proton and a
water molecule in the same step.

slow, bimolecular


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5.13Rearrangements in Alcohol Dehydration

• Sometimes the alkene product does not have the
  same carbon skeleton as the starting alcohol.

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Example
OH
H3PO4, heat


3
33
64
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Rearrangement involves alkyl group migration

• carbocation can lose a proton as shown
• or it can undergo a methyl migration
• CH3 group migrates with its pair of electrons to
  adjacent positively charged carbon

3
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Rearrangement involves alkyl group migration

CH3
CH3

• tertiary carbocation more stable

3
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Rearrangement involves alkyl group migration

CH3
CH3
3
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Another rearrangement

• CH3CH2CH2CH2OH

H3PO4, heat

mixture of cis (32)and trans-2-butene (56)
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32
Rearrangement involves hydride shift

• oxonium ion can losewater and a proton(from
  C-2) to give1-butene
• doesn't give a carbocation directlybecause
  primarycarbocations are toounstable

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Rearrangement involves hydride shift
CH3CH2CHCH3

• hydrogen migrates with its pair of electrons
  from C-2 to C-1 as water is lost
• carbocation formed by hydride shift is
  secondary

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Rearrangement involves hydride shift
CH3CH2CHCH3

mixture of cisand trans-2-butene
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Hydride Shift
H
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Carbocations can...

• react with nucleophiles
• lose a proton from the b-carbon to form an alkene
  
• rearrange (less stable to more stable)